The present invention relates to improvements made in the field of composite bearings with double ball bearings for the rotary support of a rotating shaft of a very high precision device, wherein these bearings with double ball bearings are constituted by the juxtaposition of two bearings with single ball bearings having two respective corresponding races, interior or exterior, fitting tightly axially one against the other with predetermined pre-stress.
In some very high precision devices (for example, in inertial navigation centers), it is necessary and indispensable that a rotating shaft is mounted and supported in a bore with very great precision of coaxiality without any play, in order to obtain high performances. Such a requirement necessitates that the balls of the ball bearings remain permanently in the contact of their path in order to avoid the consequences inherent in xe2x80x9ctake-offsxe2x80x9d of the balls (generation of shocks, wear and tear, appearance of plays), which are eventually reflected by a loss of precision of the device. In order to clarify ideas, it will be noted that in the case of the appearance of a vibratory operating regime, it is an effort which can for example reach 900 N which can be exerted on a ball bearing race having a diameter of the order of 35 mm and a thickness of around 3 mm.
In order to be able to meet practical requirements, one thus resorts to bearings equipped not with single ball bearings, but with double ball bearings.
The easiest solution, which would consist in using a single bearing equipped with double juxtaposed ball bearings, however, cannot be used. This is because, even if very high precision machinery were used, the double ball bearings have dimensional differences (distance between axes, diameters of the balls, diameter of the receptacles, etc.), which are too great for the rotating shaft to be able to be supported in the bore with all the required precision and absence of play.
Thus it is known that one can resort to two bearings with single ball bearings, which are paired and which are then implemented under axial pre-stress of their respective corresponding races, either interior or exterior, in such a way that they thus behave like a single bearing with double ball bearings. In this case, as opposed to the single bearing with double ball bearings, each bearing can be manufactured with all the desired precision. The respective co-operating faces of the two races, interior or exterior, of the two bearings can be rectified in order to be in perfect contact one against the other. Also, the rigidity conferred by the pre-stressing finally leads to a bearing (hereinafter referred to as xe2x80x9ccomposite bearingxe2x80x9d because it is constituted by the reunion of two bearings with mono-ball bearings), which can satisfy practical requirements with regard to precision and the absence of play of the assembly of the rotating shaft in the support bore.
Even if the results obtained with this type of composite bearing satisfy practical requirements, on the other hand the assembly processes of such a composite bearing are complex and present many problems.
A usual assembly process takes place as follows. After pairing of the composing pieces (and in particular of the two races to be placed side by side under pre-stress), the two bearings are dismounted, then a first race is implemented in support against an axial capacity; for example in the case of an interior race, it is mounted on the shaft just to the point of abutment against a radial, annular retaining wall provided on the shaft.
The second race is then pre-positioned in immediate proximity to the first race, but without being in contact with it; in the example considered of interior races, the second race is mounted in turn on the shaft in immediate proximity of the first race.
One thus brings an axial pressure tool against the second race and, with the assistance of this tool, one displaces the second race axially towards the first race just to the point of physical contact with it, then one continues to exert a force on the second race until the support pre-stress of the second race on the first reaches a pre-determined value. Still within the scope of the considered example of interior races, the tightening tool can consist in a nut which is screwed on the end of the shaft, which is threaded to this effect. This nut, which is moreover appropriately suited to be in perfect axial support against the second race, is suitable, in the course of its rotation, for displacing the second race to first of all take it against the first race, then for putting the two races under axial pre-stress at a pre-determined value.
The drawbacks of this existing process consist in that it is necessary on the one hand to specifically adjust (threading for the tightening nut) the support element of the races under pre-stress (the support bore for exterior races, the shaft for interior races) and on the other hand it is necessary to maintain in permanence the elements necessary for the assembly (threading and nut), since the pre-stress of the races is only due to the presence of the tightening tool (nut screwed on the threading for example). This results in an increase in weight of the unit and, in the case of interior races, an inertial modification of the mobile gear. These constraints may not be accepted, in particular when it is a matter of reducing as far as possible the dimensions of the device thus equipped and reducing the weight thereof, as well as when it is a matter of increasing the response rapidity performances by reducing in a maximum way the inertias of the mobile parts.
In order to attempt to overcome said drawbacks, one has proposed the combining of the second race, in support under pre-determined pre-stress against the first race, by adhering (by a resin film) the second race on its support. In this case, it is admittedly still necessary to resort to a tightening tool to place the second race and the first side by side and to exert the prestress until the adhering of the second race on its support is mechanically effective. However, the tightening tool can then be withdrawn, and it is no longer present in the completed device which is ready to function.
However, this process itself also has drawbacks. Firstly, even if the tightening tool is admittedly not maintained in the device, certain adjustments necessary to its use remain in place: thus, in the case of a nut suitable for pushing the second race, the threading realized either in the bore (for exterior races) or on the end of the shaft (for interior races) remains on the bore or on the shaft. Practice may not accept the presence of this adjustment, which is not necessary to the functioning of the device.
In particular, the adhering film for combining the second race, which is relatively thin (for example typically 1.5 xcexcm), is not very resistant to thermal shocks (for example typically variation of 3 to 4xc2x0 C. per second in an ambiance of around 70xc2x0 C.). The adhering film fractures and the races are no longer maintained under pre-stress (even becoming unstuck from their support), in such a way that the shaft is no longer supported with the required precision and the device loses its precision, or is even no longer in a state of being able to function.
In order to attempt to overcome these drawbacks, it has been proposed to increase the thickness of the layer of glue, which thus becomes able to withstand severe thermal conditions without deterioration. However, due to the very reason of the thickness of the layer of glue, the rigorous coaxiality of the races and of the support can no longer be assured when the races are put in place on the support. It is thus necessary to conserve axial positioning zones (without glue), which are suitable for assuring the mutual axial maintenance of the races and of the support and of the axial zones of different diameter (with glue), which are suitable for making the races solid with the support.
Such an assembly process necessitates manufacturing processes and proves to be too complicated.
Besides, it is to be noted that all the processes which have just been explained have the additional drawback that the pre-stressing of the two races takes place during the very course of the assembly of the second race on the support (bore or shaft), which particularly complicates the operational mode of pre-stressing.
From this viewpoint, it is thus desirable for the two races to be able to be reunited under axial pre-stress before their assembly, then to be mounted on the support in the form of a unitary block, which can thus be put into place in the same way as a traditional bearing with double ball bearings.
With this aim in view, one could admittedly envisage making the two races solid with the assistance of crimping tongs. However, this process does not prove to be very practical, and furthermore the assembled races would remain equipped with the crimping tongs, which maintain the pre-stress. This is again one of the drawbacks of the first process explained above.
It is thus an object of the invention to overcome as far as possible the drawbacks presented by the different solutions as in the state of technology and to propose an improved original solution which allows pre-assembly of the races under pre-stress, which dismisses any adaptation of the support specifically for the mounting of the races, which does not allow to remain in the completed device any additional part specific to the mounting and/or to the maintenance of the pre-stress of the races, and which allows production in relatively large series of races which are pre-assembled under pre-stress
For this purpose, according to a first embodiment, the invention proposes a composite bearing with double ball bearings such as that mentioned in the introductory clause which according to the invention is characterized in that the two races maintained fitting tightly one against the other with said pre-determined pre-stress are welded to one another by their co-operating faces.
According to a preferred embodiment, at least one of the edges, exterior or interior, of the co-operating faces of the races is beveled, and these beveled edges together form an annular groove. The welded joint is situated at the bottom of the groove and the welded joint does not project outside the groove.
Advantageously, it is the exterior edges of the co-operating faces of the races that are beveled in order to define an exterior groove and the races are made solid by a welded joint extending along the exterior edges of the faces joined end to end of the two races. However, it is also possible for the interior edges of the co-operating faces of the races to be beveled in order to define an interior groove and for the races to also be made solid by a welded joint extending along the interior edges of the faces joined end to end. Preferably, the two races are welded on the interior and on the exterior.
The welded joint can be formed from successive, continuous or discontinuous sections, on the perimeter of the groove, wherein the welded joint can in practice advantageously be a laser welded joint.
According to a second embodiment, the invention proposes a process for assembling a bearing with double ball bearings for the rotary support of a rotating shaft of a very high precision device, wherein this bearing with double ball bearings is constituted by the juxtaposition of two bearings with single ball bearings having two of their respective races, interior or exterior, axially fitting tightly together, wherein this process according to the invention is characterized in that it consists of the series of the following stages:
one pairs two bearings with single ball bearings, wherein their respective races, interior or exterior, have co-operating faces with complementary geometric properties,
after disassembly of the two bearings, one positions said two respective races against one another, and one tightens them axially against one another with a pre-determined pre-stress,
welding is carried out along at least one of the edges, exterior or interior, of the co-operating faces of the two respective races maintained under pre-stress, in such a way that the welded joint does not project radially beyond the peripheral annular faces of said races,
then one re-assembles the one-piece component formed from the two welded under pre-stress races in position on its support part and one finishes the complete assembly of the bearing by eccentration according to the principle of deep-groove ball bearings.
In a simple way, one welds the two races along the exterior edges of their co-operating faces. However, in order to homogenize the efforts being exerted between the two races, it is likewise desirable to also weld the two races along the interior edges of their cooperating faces.
In practice, the welding is achieved with the assistance of a laser beam. In the case of welding on the interior edges of the races, the laser beam is guided onto the interior edges of the co-operating faces of the races successively through holes of a component for support and centering of the races positioned within them, and the welding is carried out in successive sections. By reason of the passage of the laser beam through said holes, the laser beam cannot be displaced in a continuous way, which means that the welding is realized by discontinuous, successive sections.
Preferably, the edges of the respective races are beveled and the welding is effected in the bottom of the groove formed by two co-operating beveled edges, in such a way that no welded joint element projects and the peripheral lateral face of the assembled races remains smooth. Nothing hampers their assembly on the support (bore or shaft).
Advantageously, the two races being welded along their interior and exterior co-operating edges, the welding of the interior co-operating edges is effected firstly and the welding of the exterior co-operating edges secondly.
According to a third embodiment, the invention proposes a tool of support, of coaxial centering and of pre-stressing of two races of ball bearings destined to be welded to one another, in order to implement said process.
According to a first method of realization, a tool designed to handle two interior races is proposed, wherein this tool, being constructed according to the invention, is characterized in that it comprises
an essentially tubular chuck having a lateral wall with a cylindrical external face of revolution, which is suitable for supporting and centering coaxially, by their internal faces, two interior races positioned one after the other,
first solid means of axial abutment of said chuck, against which a free face of one of the two races comes in support,
and coaxial means of tightening to said chuck, comprising second means of axial abutment which are suitable for coming in support against the free face of the other of the two races in such a way that under the effect of said tightening means, the two races are tightened against one another until a pre-determined pre-stress value is obtained.
Advantageously in this case, the tool comprises a support plate of the chuck, wherein said plate is in the form of a bowl suitable for receiving coaxially a hub. The first means of abutment are provided on or constituted by the peripheral edge of the lateral wall of said hub. The second means of abutment are provided on a solid cover of said chuck, wherein said cover is open centrally to give access to the interior of the chuck.
In a second method of realization of the tool, a tool designed to handle two exterior races is proposed, wherein this tool, being constituted according to the invention, is characterized in that it comprises:
a hub of which the lateral wall presents a cylindrical internal face of revolution suitable for supporting and centering axially, by their external faces, two exterior races positioned one after the other, wherein the lateral wall of said hub is provided with a multiplicity of traversing holes distributed peripherally and in relation to which the external edges to be welded of the two races are situated, wherein each hole constitutes a passage so that a laser beam crossing it can weld said external edges of the two races,
an approximately tubular chuck supported at the interior of the hub and having a lateral wall situated in relation to the wall of the hub,
first solid means of axial abutment of said hub, against which a free face of one of the two races comes in support,
and coaxial means of tightening to said hub, comprising second means of axial abutment suitable for coming in support against the free face of the other of the two races, in such a way that under the effect of said means of tightening, the two races are tightened against one another until a pre-determined pre-stress value is obtained.
Advantageously in this case, the second means of abutment are solid in respect of the chuck and are constituted by a flank projecting radially towards the exterior from the upper edge of the lateral wall of said chuck.
Advantageously, the tool has means of rotary support of the chuck.
Preferably, in one or the other of said tools, the lateral wall of the chuck is equipped with a multiplicity of traversing holes distributed peripherally and in relation to which the edges, respectively interior and exterior, of the co-operating faces of the two races to be welded are situated, wherein each hole constitutes a passage so that a laser beam crossing it can weld the edges of the races appearing at the opening of this passage. It is thus desirable for the holes of the chuck to be inclined towards the foot of the interior towards the exterior of the chuck wall in order to facilitate the positioning of the welding laser beam approximately in the axis of each hole.